The aura is our energy field/life force that surrounds our body and all living things, it;s shaped like an egg and made up of fibrous light, it is commonly known has having 7 layers, most psychics will see only the first 3 of them these are the ones most closest to our physical body, the inner layers being mind, health and emotions, outer layers are the soul,spirit.
It's our energy field, life force and is generaly seen as a light around the head area.

Can people see the color of someone's aura. My friend said she use to be able to see. She said when I was happy my aura was bright, but when I was stressed it was a dark color. I can sense how somebody is by being around them. If somebody is a real negative person I can sense that as soon as they walk in the room. It's hard to explain to people, but I don't know anybody else who can sense this about people. It's really hard to ignore. What does this mean?

Sure, anyone can see anothers aura along with thier own, it just takes practice. As for what ur feeling, it sounds like ur picking up on their energy, or at least something of empath ability perhaps where u can feel what others feel. Should try studying up on both auras and empath I'd say, might give u the answers ur looking for.